Image forming apparatus and image forming method

ABSTRACT

There is provided an image forming apparatus that is provided with photosensitive drums ( 11 M), ( 11 C), ( 11 Y) and ( 11 Bk) for forming latent images by irradiation of light reflected by a polygon mirror and a fixing device ( 40 ) for fixing toner images corresponding to the latent images on a sheet material to prevent deterioration of an image to be formed if reflective surfaces of a reflective polyhedron are thinned out in reducing a process speed of image formation. This apparatus is further provided with a control unit ( 35 ) for, if the number of surfaces of the polygon mirror is m (m is an integer of three or more), performing control for selecting n such that m/(n+1) does not make an integer as a relation between m and n and performing control for making n surfaces unused as a reflective surface of light among continuous n+1 surfaces of the reflective polyhedron in the decelerating image forming process.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image forming apparatus suchas a copying machine and a printer. More specifically, the presentinvention relates to an image forming method that is applied to theapparatus and relates to an apparatus for performing image formation byconverting light to scanning light using a reflective polyhedron.

[0003] 2. Description of the Related Art

[0004] Conventionally, there have been known two methods as an imageforming method for obtaining a color image on a sheet material bysuperimposing toner images of a plurality of colors. One is a method offorming electrostatic latent images on an image bearing member,sequentially developing them by toner to transfer them to a sheetmaterial each time they are developed and superimposing toner images ofa plurality of colors on the sheet material (hereinafter referred to asmultiple transfer method). The other is a method of superimposing tonerimages of a plurality of colors on an image bearing member or anintermediate transfer member and transferring the superimposed tonerimages of a plurality of colors collectively to a sheet material(hereinafter referred to intermediate transfer method).

[0005] Among them, the former method is put to practical use withstructures described in DE2607727, Japanese Patent Application Laid-openNo. Sho 50-50935 and the like.

[0006] On the other hand, the latter method is a technology havingadvantages such as a simple structure and currently drawing attention,although it has problems of color mixture at the time of development andimage deterioration at the time of re-transfer.

[0007] Moreover, in recent years, a so-called tandem type multipletransfer method, which is capable of forming a full color image usingone path by providing process units such as a photosensitive member anda developing device independently for each color, begins to drawattention in terms of high-speed productivity.

[0008] In any of the methods, toner images T of a plurality of colorsformed and superimposed on a sheet material 103 are finally heated andfixed by a fixing device that uses a roller or a belt as shown in FIG.8. As a result, toner images of each color are mixed and a full colorimage can be obtained. FIG. 8 is a schematic view of a fixing devicethat is used in the conventional image forming apparatus.

[0009] An example of a fixing device of this type by a heat rollermethod will be hereinafter described with reference to FIG. 8. In FIG.8, a fixing roller 101 is for melting to fix the toner T on the sheetmaterial 103 by heating and application of pressure. A pressure roller102, which is disposed opposing the fixing roller 101, is for applyingpressure to the fixing roller 101.

[0010] Usually, the fixing roller 101 often has a structure to be heatedby a heater 104 such as a halogen lamp from the inside of the roller.

[0011] Therefore, it is a general practice to monitor a surfacetemperature of the fixing roller 101 by a temperature sensing element114 such as a thermistor to on/off control the heater 104, therebyperforming temperature control.

[0012] In addition, as the toner T, toner of four colors of yellow,cyan, magenta and black is generally used.

[0013] In this case, silicone oil 106 or the like is applied to thesurface of the fixing roller 101 by a blade 105 via a drawing roller 116and an applying roller 115 in order to prevent offset. Alternatively,toner originally containing wax or oil component is used to preventoffset.

[0014] In addition, the surface of the fixing roller 101 may be cleanedby a cleaning web 107 or the like, if necessary.

[0015] Incidentally, in the case in which a transparent film for anover-head projector (hereinafter referred to as OHP (over-headprojector) sheet), a sheet for realizing high gloss (hereinafterreferred to as high gloss paper or high gloss sheet) or the like isused, a method of giving it a heat quantity larger than that requiredfor fixing an image on a normal transfer sheet to perform fixing andrealizing desired transparency and glossiness is often used.

[0016] As a method for this purpose, for example, it is possible toincrease a heat value of the heater 104 incorporated in the fixingroller 101. However, with this method, a sufficient heat quantity is notobtained in many cases and it takes long to make a temperature of thefixing roller 101 follow. Thus, this method cannot always be effective.

[0017] As another method, there is a method of reducing a fixing speedby the fixing roller 101 to give a sufficient heat quantity to a sheet.With this method, a desired heat quantity is easily obtained and, at thesame time, it is possible to execute fixing immediately withoutproviding a waiting time because it is unnecessary to change temperaturesetting of the fixing roller 101 itself.

[0018] However, if reduction in a fixing speed is attempted as describedabove, a sheet exists on a transfer portion and a fixing portionsimultaneously unless a distance between the transfer unit and thefixing unit is sufficiently large as compared with a maximum sheetlength of a sheet to be used (in case of an intermediate transfermember, unless a distance between a primary transfer unit and asecondary transfer unit is sufficiently large as compared with themaximum sheet length) in case of the above-mentioned multiple transfermethod.

[0019] Consequently, a fixing speed cannot be reduced after toner imagesof final colors are formed on a photosensitive member until all tonerimages are transferred to the sheet.

[0020] That is, it has become necessary to additionally rotate atransfer drum or an intermediate transfer member. In this case, aprinting time is substantially extended

[0021] Moreover, in recent years, a so-called quadruple multipletransfer method using photosensitive members for four colors drawsattention as a high-speed full color printing method. In this method,decelerated fixing cannot be performed with a machine in which adistance between portions for transferring and fixing final color isshorter than a maximum sheet length unless the machine is put in adecelerated state in advance from the time when an image is formed on aphotosensitive member. (It is needless to mention that, in theabove-mentioned multiple transfer or intermediate transfer membermethod, it is also possible to perform the same method as describedbelow instead of rotating the transfer drum or the intermediate transfermember additionally.)

[0022] That is, in an image forming apparatus with a short distancebetween a transfer unit and a fixing unit, it becomes necessary toperform image formation in a decelerated state in advance in order toavoid shock caused by reducing a fixing speed during image formation.

[0023] Nevertheless, the applicant of this application and the likefound, as a result of examining a laser printer of a method ofdeflecting a laser beam irradiated from a light source by a reflectivepolyhedron (e.g., polygon mirror) to scan a member to be scanned such asa photosensitive member by the deflected laser beam, that a laser beamquantity became excessive and deficiencies such as tendency of imageblock-up and defective middle tones were caused if image formation isperformed in a decelerated state (e.g., at a half speed of a standardspeed) in the laser printer.

[0024] Thus, it became necessary to reduce a laser beam quantity in thedecelerated state. However, it was difficult to steadily realize asignificant change of a light amount such as reducing it to half or onethird as in this example using a laser beam emitting circuit.

[0025] In addition, it took long to change a speed of a polygon mirrorand a process speed could not be changed promptly.

[0026] There is known a method of making predetermined surface unusedamong all surfaces of a polygon mirror instead of reduction of a laserbeam quantity.

[0027] This method is a so-called thinning-out scanning for thinning outthe number of surfaces to be used of a polygon mirror to use them andsubstantially reducing a laser beam quantity, which is recognized as apublicly-known technology. Further, thinning out one surface ofreflective surfaces of a polygon mirror means that a reflective surfaceskipped one surface from a reflective surface used for optical scanningis used for the next optical scanning.

[0028] However, when the applicant of this application or the likeperformed this thinning-out scanning, a new problem concerning an imageoccurred.

[0029] The problem is that dispersion of scanning lengths in the mainscanning direction that each surface of a polygon mirror has withrespect to an image having periodicity such as a dither image, so-calledjitter, causes interference and fringe-like streak unevenness occurs on,for example, uniform images of middle tones.

[0030] This moiré phenomenon will be described with reference to FIGS. 4to 7. FIG. 4 is a schematic view showing a relation between a structureof a polygon mirror to be used in the conventional image formingapparatus and scanning lengths. FIG. 5 is a conceptual view showing adither matrix to be used in the conventional image forming apparatus.FIGS. 6 and 7 are schematic views showing image patterns to be formed inthe conventional image forming apparatus.

[0031] As an example, the case in which a polygon mirror 121 has eightsurfaces as shown in FIG. 4A will be described.

[0032] If a rotating shaft 123 of the mirror deviates with respect to acentral position 122, influence in the main scanning direction appearsas a infinitesimal deviation (undulation) of a scanning position fromleft to right of FIG. 4B as shown by scanning lines 11 to 18corresponding to mirror surfaces S1 to S8 of FIG. 4B.

[0033] Here, starting positions for starting writing in the left end arealigned by synchronizing writing signals by known detecting means.However, in accordance with the rightward scanning, deviation ofpositions occurs for each surface of the polygon mirror, which becomesmaximum in the right end.

[0034] That is, as the scanning moves from the left end to the rightend, dots gradually deviate and the eight ends of the scanning undulateto left and right at a period of every eight scanning lines.

[0035] On the other hand, as an example of an image pattern, variousdither matrix as shown in FIG. 5 are assumed and pixels are grown in anorder indicated by numerical values (i.e., grown in a swirling shapeoutward from the center).

[0036] In this case, in printers of recent years, a so-called multiplevalue dither for further dividing the inside of one dot in the figureinto a plurality of steps to grow pixels may be used.

[0037] In any case, when such a repeating pattern is used, growthcenters as shown in FIG. 6 (represented by white circles or blackcircles) appear on a bit map.

[0038] Here, a few dither matrixes D based on the growth centers(represented by white circles or black circles) are shown as an examplein FIG. 6.

[0039] When undulation of a beam position appearing at a period of everyeight scanning lines due to the above-mentioned deviation of polygonsurfaces is caused in this example, sparse parts and dense parts appearat a period of every eight scanning lines with approaching the right endon the image. Then, for example, the part of the black growth centercorresponding to the scanning line 11 is enhanced in FIG. 6 (looksslightly darker than the other growth centers).

[0040] The parts of the black circles shown in FIG. 6 look black becausethe part with less vertical deviation has a closer distance betweenupper and lower dots than the other parts due to undulation in imageformation of the polygon mirror shown in FIG. 4.

[0041] In other words, it can be said that the parts of the whitecircles of FIG. 6 are parts in which a distance between upper and lowerdots was expanded (in a slant direction) at the time of image formationdue to the undulation of the polygon mirror shown in FIG. 4 and the dotsbecame sparse, as a result, the entire pixel relatively looks light.

[0042] It seems that the pixels look dark when each dot is concentratedand look light when each dot is dispersed because an amount of toner ora shape of a toner image is affected by a change of a depth of potentialat the time of latent image formation by the electrophotographic systemor an edge effect at the time of development.

[0043] In any case, shades of a pixel is caused by undulation in themain scanning direction by a polygon mirror of FIG. 4 and the shadesoccurs at a period of this undulation.

[0044] However, as in this example, if the period of undulation (here,period of eight lines) is large as compared with a size of a dithermatrix, since a distance between dots to be enhanced expands andneighboring enhanced dots do not look connected, a moiré pattern israrely discerned on an image.

[0045] Nevertheless, when thinning-out scanning of a polygon mirrorfollowing reduction in an image forming speed as described above isperformed, for example, if every other surface of a polygon mirror isused as the image forming speed is reduced to half, the above-mentionedundulation appears at a period of four scanning lines corresponding tohalf of the eight surfaces.

[0046]FIG. 7 shows how interference between a dither growth center andundulation occurs in this case. In FIG. 7, white circles and blackcircles are also centers of the dither matrixes shown in FIG.. 5. Thatis, in FIG. 7, the white circles and the black circles are also centersof the dither matrixes D shown in FIG. 6.

[0047] As is evident from FIG. 7, as the period of undulation is reducedto four scanning lines, a distance between dots to be enhanced isnarrowed and strong correlation as shown by L1 to L3 (i.e., a slantfringe pattern due to moiré is caused.

[0048] A degree of this moiré fringe pattern changes according to amagnitude of undulation of scanning by a polygon mirror, a scanningdensity for one dot, a shape of dither and the like. As an example, itwas found that, when the scanning density was set to 600 dots/inch (inthis case, a pixel by the above-mentioned dither was equivalent toapproximately 120 lines), an interference fringe in the right end of thescanning by the polygon mirror was a slant line with a pitch ofapproximately 0.6 mm and, then, even if a manufacturing accuracy of apolygon mirror and a rotating shaft was increased to suppress adifference between a maximum value and a minimum value of an amount ofdeviation of dots in the main scanning direction causing undulation tobe equal to or less than 1 dot, a moiré fringe at the time ofthinning-out scanning due to deceleration was very conspicuous.

[0049] On the other hand, it was found that it was difficult in terms ofmanufacturing accuracy to reduce undulation caused by a polygon mirrorto a degree at which a moiré fringe was not seen (e.g., 1/5 dot or lessin the right end).

[0050] Further, this moiré fringe pattern tends to be conspicuous when asize of a dither (the number of dots in one side) and the number ofsurfaces of a polygon mirror are close. Since the number of lines of adither that is often used in general is 100 to 200 (i.e., the number ofdots in one side is 6 to 3 dots) and, on the other hand, a polygonmirror with four to twelve surfaces is often used, if polygon mirrorsurface is thinned out and used as described above, the moiré fringepattern changes to be conspicuous.

SUMMARY OF THE INVENTION

[0051] The present invention has been devised in view of the above andother drawbacks, and it is an object of the present invention to providean image forming apparatus and an image forming method that are capableof preventing decrease in a quality of an image to be formed even ifreflective surfaces of a reflective polyhedron are thinned out inreducing an image forming speed.

[0052] Another object of the present invention is to provide an imageforming apparatus comprising: a movable image bearing member; a lightsource; a rotatable reflective polyhedron for reflecting light from saidlight source and optically scanning said image bearing member; a firstmode for forming an image on said image bearing member at a first speed;and a second mode for forming an image on said image bearing member at aspeed of 1/(n+1) of the first speed, and when it is assumed that thenumber of surfaces of said reflective polyhedron is m, m/(n+1) is not aninteger at the time of said second mode (provided that m is an integerof three or more and n is a natural number).

[0053] Still another object of the present invention is to provide animage forming method comprising the steps of: switching from a firstmode for forming an image on an image bearing member at a first speed toa second mode for forming an image on an image bearing member at a speedsmaller than the first speed; and using reflective surfaces of areflective polyhedron, in which the number of rotatable surfaces is m,for reflecting light from a light source and optically scanning an imagebearing member by skipping n surfaces such that m/(n+1) does not make aninteger (provided that m is an integer of three or more and n is anatural number).

[0054] Further objects of the present invention will become apparentfrom the following explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In the accompanying drawings:

[0056]FIG. 1 is a sectional view of a full color image forming apparatusof a four-drum system of an electrophotographic method that is a firstembodiment of an image forming apparatus in accordance with the presentinvention;

[0057]FIG. 2 is a structural view of a polygon mirror 41 that is used inthe image forming apparatus shown in FIG. 1;

[0058]FIG. 3 is a sectional view of a full color image forming apparatusby an intermediate transfer member method with one drum four developingdevices of an electrophotographic method that is a second embodiment ofthe image forming apparatus in accordance with the present invention;

[0059]FIG. 4 is a schematic view showing a relation between a structureof a polygon mirror to be used in the conventional image formingapparatus and scanning lengths;

[0060]FIG. 5 is a conceptual view showing a dither matrix to be used inthe conventional image forming apparatus;

[0061]FIG. 6 is a schematic view showing an image pattern to be formedin the conventional image forming apparatus;

[0062]FIG. 7 is a schematic view showing an image pattern to be formedin the conventional image forming apparatus; and

[0063]FIG. 8 is a schematic view of a fixing device to be used in theconventional image forming apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064] Preferred embodiments of the present invention will behereinafter described in detail with reference to the accompanyingdrawings. Further, dimensions, materials and shapes of components andtheir relative arrangements described in the embodiments are notintended to limit the scope of the present invention to them unlessspecifically described otherwise.

[0065] In addition, in the accompanying drawings, like referencenumerals designate the same or similar parts throughout the figuresthereof.

[First Embodiment]

[0066] First, a first embodiment of an image forming apparatus inaccordance with the present invention will be described with referenceto FIGS. 1 and 2. FIG. 1 is a sectional view of a full color imageforming apparatus of a four-drum system of an electrophotographic systemthat is a first embodiment of an image forming apparatus in accordancewith the present invention.

[0067] The image forming apparatus shown in FIG. 1 has a plurality ofimage forming units M, C, Y and Bk and is provided with a transfer belt30, which is disposed traversing each image forming unit, as conveyingmeans for conveying a recording material.

[0068] Cylindrical photosensitive drums (photosensitive members) 11M,11C, 11Y and 11Bk as movable image bearing members are supportedrotatably in an arrow a direction on the image forming units M, C, Y andBk, respectively. Reference numerals 12M, 12C, 12Y and 12Bk denoteprimary charger, which are disposed a predetermined distance apart fromthe photosensitive drums 11M, 11C, 11Y and 11Bk, respectively.

[0069] Reference numerals 13M, 13C, 13Y and 13Bk denote laser exposuredevices as exposing means having a light source and a polygon mirror asa reflective polyhedron which is rotatable for reflecting light from thelight source to optically scan an image bearing member. The laserexposure devices 13M, 13C, 13Y and 13Bk expose the photosensitive drums11M, 11C, 11Y and 11Bk according to image signals 23M, 23C, 23Y and 23Bkdownstream in the rotating direction of the photosensitive drums withrespect to the primary chargers 12M, 12C, 12Y and 12Bk, respectively.

[0070] Reference numerals 14M, 14C, 14Y and 14Bk denote developingdevices containing toner of magenta, cyan, yellow and black,respectively, which are disposed opposing the photosensitive drums 11M,11C, 11Y and 11Bk further downstream than the exposure devices of thephotosensitive drums.

[0071] Reference numeral 30 denotes a transfer belt for carrying a sheetmaterial P that is a recording material supplied from a registrationroller 20. The transfer belt 30 is stretched and suspended (a state inwhich it is suspended while being applied tension) by a driving roller31 and a separation roller 32 and is driven in an arrow b directioncontacting the photosensitive drums 11M, 11C, 11Y and 11Bk.

[0072] Transfer chargers 15M, 15C, 15Y and 15Bk are disposed in eachtransfer position against each of the photosensitive drums 11M, 11C, 11Yand 11Bk from the upstream in the moving direction of the transfer belt30 so as to nip the transfer belt 30 between respective transferchargers and photosensitive drums.

[0073] Cleaners 16M, 16C, 16Y and 16Bk are cleaners for removingresidual toner on each of the photosensitive drums 11M, 11C, 11Y and11Bk and belt cleaner 33 is a belt cleaner for cleaning the transferbelt 30.

[0074] A fixing device 40 is fixing means for fixing a toner image onthe sheet material P to which the toner image is transferred.

[0075] A control unit 35 is control means for controlling operations ofthe entire image forming apparatus and controls operations of thephotosensitive drums 11M, 11C, 11Y and 11Bk, the laser exposure devices13M, 13C, 13Y and 13Bk, the fixing device 40 and the like.

[0076] Operations of the image forming apparatus configured as describedabove will be described with an image forming unit M as an example.

[0077] The photosensitive drum 11M has an optical semiconductive layeron a surface of a conductive base such as aluminum and rotates in thearrow a direction.

[0078] The photosensitive drum 11M is negatively charged on its surfaceuniformly by the primary charger 12M and, then, subject to exposure bythe laser exposure device 13M, whereby an electrostatic latent imagecorresponding to image data is formed.

[0079] The exposure by the laser exposure device 13M is performed usinga polygon mirror as described later.

[0080] The developing device 14M performs development using toner thatis negatively charged and forms a toner image corresponding to theelectrostatic latent image on the surface of the photosensitive drum11M.

[0081] Then, the toner image formed on the surface of the photosensitivedrum 11M is transferred to the sheet material P, which is suppliedtoward the transfer belt 30 from the registration roller 20 insynchronous with the toner image being formed, by the transfer roller15M.

[0082] On the other hand, the photosensitive drum 11M after the tonerimage is transferred therefrom has residual toner attached to thesurface thereon, which is removed by the cleaner 16M and the next imageis formed thereon.

[0083] The above-mentioned operations are performed in each imageforming unit at predetermined timing and toner images formed on eachphotosensitive drum are transferred to the sheet material P held by thetransfer belt 30 one after another.

[0084] In the case of a full color mode, toner is transferred to thesheet material P in the order of M, C, Y and Bk. In the case of a singlecolor mode or a two or three color mode, toner of necessary colorsmultiply transferred to the sheet material P in the above-describedorder in the same way.

[0085] The sheet material P to which toner images are transferred issupplied to the fixing device 40 by the transfer belt 30 and is subjectto heating and application of pressure to be fixed, whereby a full colorimage is obtained.

[0086] Further, the transfer belt 30 that has finished supplying thesheet material P to the fixing device 40 has its surface cleaned by thebelt cleaner 33.

[0087] In such an apparatus as described above, when a distance A from afixing nip to a transfer nip of the fourth color is shorter than a sheetmaterial, if a sheet material such as an OHP sheet, high gloss paper(high gloss sheet) or cardboard is used instead of plain paper, it ispreferable to reduce a speed itself of an image forming process of thefirst color in advance to be slower than a speed (first speed) of astandard image forming process (first image forming mode) to have aspeed (second speed) of a deceleration image forming process (secondimage forming mode) as a fixing speed is reduced to half or one third inorder to secure transparency, glossiness, fixing property and the like.That is, if the fixing speed is reduced in the second mode to half ofthe fixing speed in the first mode, both speeds of an image bearingmember and fixing means are reduced to half of the speeds in the firstmode.

[0088] In this case, if the fixing speed is reduced to half using apolygon mirror of eight surfaces or is reduced to one third using apolygon mirror of six surfaces, an interference fringe tends to occurbetween the polygon mirror and a pixel such as a dither as described inthe section of the conventional art. In order to solve this problem, inthis embodiment, polygon mirrors of seven surfaces are used for all thelaser exposure devices 13Bk, 13Y, 13C and 13M that are optical systems.

[0089] In this way, as shown in FIG. 2, for example, when a processspeed is reduced to half, if thinning-out scanning is performed forevery one surface in the order of 1 to 7 in a polygon mirror 41, aperiod until the identical surface appears can be made seven(1→3→5→7→2→4→6→1) FIG. 2 is a structural view of the polygon mirror 41,which is used in the image forming apparatus shown in FIG. 1.

[0090] As a result, a period of undulation of scanning lines can avoidbeing shortened by thinning-out scanning and deterioration of aninterference fringe can be prevented.

(Generalization)

[0091] The above description of this embodiment is generalized asfollows. That is, in general, when it is assumed that the number ofsurfaces of a polygon mirror is m (provided that m is an integer ofthree or more) and a deceleration ratio by thinning-out scanning for nsurfaces is 1/(n+1) (e.g., each scanning is performed by thinning outevery other surface, a speed at the time of deceleration (deceleratingimage forming process) is 1/(n+1) ½), m/(n+1) is adjusted not to be aninteger. (n is a natural number.)

[0092] That is, deterioration of an interference fringe at the time ofdeceleration can be prevented by adjusting a relation between m and nsuch that m/(n+1)≠integer is satisfied.

[0093] That is, the image forming method in the image forming apparatusof this embodiment has a step of switching from the first mode forforming an image on an image bearing member at the first speed to thesecond mode for forming an image on the image bearing member at a speedlower than the first speed and a step of using reflective surfaces of arotatable reflective polyhedron, in which the number of surfaces forreflecting light from a light source and optically scanning an imagebearing member is m, to scan optically by skipping n surfaces such thatm/(n+1) does not make an integer (provided that m is an integer of threeor more and n is a natural number).

[0094] However, if m/(n+1) is adjusted not to be an integer, it is morefavorable to adjust m/(n+1) to be indivisible.

[0095] For example, deterioration of an interference fringe can beprevented by thinning-out one, two and three surfaces respectivelyaccording to the deceleration ratios of half, one third and one quarterif polygon mirrors of eleven, seven or five surfaces is used.

[0096] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of ten surfaces isused.

[0097] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two and four surfaces respectively according tothe deceleration ratio of one third and one fifth if polygon mirrors ofeight surfaces is used.

[0098] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of four surfaces isused.

[0099] Further, as timing for starting deceleration of a process speed,in the case of the four-drum system as in this example, all speeds maybe put in a decelerated state in advance prior to supplying the sheetmaterial P. In addition, if there is a device for automaticallydetecting an OHP sheet (OHP paper) and the like, an image may be formedby changing a process speed to a decelerated state after the sheetmaterial P is supplied to the registration roller 20 at a normal speedand stopped temporarily and, then, it is detected by a detecting device22 that the sheet material P is, for example, an OHP sheet(transparent).

[0100] In this way, automatic detection of special paper such as an OHPsheet can be realized and throughput can be maximized.

[0101] Further, it is needless to mention that, in the above-mentionedexample, at the time of reducing a fixing speed to 1/(n+1), n surfacesof a polygon mirror is thinned out, whereby it is sufficient to outputimage signals 23M to 23Bk without changing a basic clocks of the imagesignals, therefore by thinning out the image signals themselves by nlines for each (n+1) line. Thus, load applied to a controller can bereduced.

[0102] As described above, this embodiment is for providing a pluralityof exposing means, reflective polyhedrons and image bearing members toform toner images of different colors, respectively. Therefore, theabove-mentioned deficiencies of an image at the time of deceleration ofa color image forming apparatus by the tandem type multiple transfermethod can be prevented.

[Second Embodiment]

[0103] A second embodiment of the image forming apparatus in accordancewith the present invention will be described with reference to FIG. 3.FIG. 3 is a sectional view of a full color image forming apparatus by anintermediate transfer member method with one drum and four developingdevices of an electrophotographic system, which is the second embodimentof the image forming apparatus in accordance with the present invention.

[0104] In FIG. 3, reference numeral 62 denotes a laser exposure deviceas exposing means having inside a polygon mirror 52 as a reflectivepolyhedron being an element of the present invention. The laser exposuredevice 62 exposes the surface of a photosensitive drum 61 as an imagebearing member, which is uniformly charged by primary charging means 53,in accordance with an image signal 51.

[0105] Reference numeral 63, 64, 65 and 66 denotes developing devices ofa rotary type that rotate in an arrow direction one after another todevelop the photosensitive drum 61. Reference numeral 55 denotes anintermediate transfer belt as intermediate transferring means thatsuperimposes each image of Y, M, C and Bk one after another on itssurface to bear the images.

[0106] Reference numeral 67 is a primary transferring device from thephotosensitive drum 61 to the intermediate transfer belt 55. Referencenumeral 56 denotes a secondary transferring device for collectivelytransferring images on the intermediate transfer belt 55 to the sheetmaterial P, which is supplied to the position between the secondarytransferring device 56 and the intermediate transfer belt 55 via a sheetsupplying roller 69, a registration roller 59 and the like.

[0107] The sheet material P having the images collectively transferredto its surface is subject to heating and fixing by the fixing device 53as fixing means being an element of the present invention.

[0108] In the embodiment as described above, there is a case in which adistance B between a primary transfer nip and a secondary transfer nipis smaller than a maximum paper size of paper that is an object ofprinting in a decelerated state. In this case, it is preferable toperform at least image formation and primary transfer of a final color(Bk in this case) in a decelerated state in advance, or to cause theintermediate transfer belt 55 to turn idly once more after primarytransfer of a final color is finished and cause at least theintermediate transfer belt 55 and the secondary transferring device 56to decelerate during this idle turning and, thereafter performdecelerated fixing by the fixing device 53.

[0109] Naturally, since throughput is decreased with the latter method,it is favorable to decelerate a process speed after primary transfer ofa third color and before primary transfer of a fourth color with theformer method in order to obtain maximum throughput.

[0110] In such a case, when it is assumed that the number of surfaces ofthe polygon mirror 52 is m and a deceleration ratio by thinning-outscanning for n surfaces is 1/(n+1), m/(n+1) is adjusted not to be aninteger, that is, m/(n+1)≠integer, whereby deterioration of aninterference fringe at the time of deceleration of the fourth color canalso be prevented.

[0111] However, if m/(n+1) is adjusted not to be an integer, it is morefavorable to adjust m/(n+1) to be indivisible.

[0112] For example, deterioration of an interference fringe can beprevented by thinning-out one, two and three surfaces respectivelyaccording to the deceleration ratios of half, one third and one quarterif polygon mirrors of eleven, seven or five surfaces is used.

[0113] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of ten surfaces isused.

[0114] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two and four surfaces respectively according tothe deceleration ratio of one third and one fifth if polygon mirrors ofeight surfaces is used.

[0115] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of four surfaces isused.

[0116] In addition, since a toner image is transferred to a sheetmaterial using intermediate transferring means, miniaturization and costreduction in an apparatus can be realized.

[0117] Further, in this second embodiment, if a device for detecting asheet material 58 or the like as detecting means is used as described inthe first embodiment, a type of a sheet material after it is suppliedcan be automatically detected and, if it is an OHP sheet, the imageforming apparatus can enter a deceleration mode of half or one thirdafter image formation and primary transfer of first to third colors arefinished and before image formation of a fourth color.

[0118] Further, the case in which the intermediate transfer belt 55 isused as an intermediate transfer member is described in this example, itis need less to mention that the description is also applied to a casein which an intermediate transfer drum or the like is used.

[Third Embodiment]

[0119] In the second embodiment, the case of the intermediate transfermethod using the intermediate transfer belt 55 is described. In amultiple transfer method for sequentially developing latent images on anonly photosensitive drum to transfer the latent images to a sheetmaterial each time they are developed, if a distance between a transferunit and a fixing unit is shorter than a length of a maximum sheetmaterial, it is also preferable to reduce a process speed after imageformation of a third color is finished and before image formation of afourth color in order to increase throughput at the time whendecelerated fixing is executed.

[0120] In such a case, as in the second embodiment, when it is assumedthat the number of surfaces of the polygon mirror 52 as a reflectivepolyhedron is m and a deceleration ratio by thinning-out scanning for ton surfaces is 1/(n+1), m/(n+1) is adjusted not to be an integer, thatis, m/(n+1)≠integer, whereby deterioration of an interference fringe atthe time of deceleration of the fourth color can also be prevented.

[0121] For example, deterioration of an interference fringe can beprevented by thinning-out one, two and three surfaces respectivelyaccording to the deceleration ratios of half, one third and one quarterif polygon mirrors of eleven, seven or five surfaces is used.

[0122] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of ten surfaces isused.

[0123] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two and four surfaces respectively according tothe deceleration ratio of one third and one fifth if polygon mirrors ofeight surfaces is used.

[0124] In addition, other than the above-mentioned embodiment,deterioration of an interference fringe can be prevented by, forexample, thinning-out two surfaces respectively according to thedeceleration ratio of one third if polygon mirrors of four surfaces isused.

[0125] As described above, according to the present invention, in adecelerated image forming process for reducing an operation speed of animage bearing member and fixing means to 1/(n+1) of an operation speedof the image bearing member and the fixing means in case of a standardimage forming speed, if the number of surfaces of a reflectivepolyhedron is m (provided that m is an integer of three or more),control means performs control for selecting n (integer) such thatm/(n+1) does not make an integer as a relation between m and n and, atthe same time, performs control for making n surfaces unused as areflective surface of light among continuous n+1 surfaces of areflective polyhedron. As a result, for example, if the number ofreflective surfaces of the reflective polyhedron is seven and anoperation speed is made ½, an order of the reflective surfaces becomes1, 3, 5, 7, 2, 4, 6, 1, 3 . . . , whereby a period of deviation in thescanning direction can be prevented from becoming short and theabove-mentioned deficiencies of an image at the time of deceleration canbe prevented.

[0126] Here, making n surfaces unused as a reflective surface of lightamong continuous n+1 surfaces of a reflective polyhedron means that, forexample, if a polygon mirror of seven surfaces is used as a reflectivepolyhedron and n=2, each surface of this polygon mirror is ordered as 1,2, 3, 4, 5, 6 and 7 and two surfaces out of three surfaces are madeunused continuously as a reflective surface.

[0127] That is, in the above example, this means selecting thereflective surfaces in the order of 1, 4, 7, 3, 6, 2, 5, 1, 4, 7 . . .This is the same for a reflective polyhedron of other numbers ofsurfaces and an arbitrary n.

[0128] Thus, it is seen that an image forming apparatus and an imageforming method are provided. One skilled in the art will appreciate thatthe present invention can be practiced by other than the preferredembodiments which are presented for the purposes of illustration and notof limitation, and the present invention can be modified in any waywithin the technical thoughts of the present invention.

What is claimed is:
 1. An image forming apparatus comprising: a movableimage bearing member; a light source; a rotatable reflective polyhedronfor reflecting light from said light source and optically scanning saidimage bearing member; a first mode for forming an image on said imagebearing member at a first speed; and a second mode for forming an imageon said image bearing member at a speed of 1/(n+1) of the first speed,wherein, when it is assumed that the number of surfaces of saidreflective polyhedron is m, m/(n+1) is not an integer at the time ofsaid second mode (provided that m is an integer of three or more and nis a natural number).
 2. An image forming apparatus according to claim1, wherein a reflective surface of said reflective polyhedron that is nsurfaces skipped from a reflective surface used for optical scanning isused for the next optical scanning at the time of said second mode. 3.An image forming apparatus according to claim 1, further comprisingmovable fixing means for fixing a toner image on a recording material towhich the toner image formed on said image bearing member is transferredwhile conveying the recording material, wherein a moving speed at thetime of said second mode of said image bearing member and said fixingmeans is 1/(n+1) of a moving speed at the time of said first mode.
 4. Animage forming apparatus according to claim 3, wherein, if the recordingmaterial is a recording material having translucency, a rather thickrecording material or a recording material for improving a glossiness ofan image, said second mode is executed.
 5. An image forming apparatusaccording to claim 3, further comprising detecting means for detecting atype of a recording material, wherein timing for executing said secondmode is determined based on a detection result of said detecting means.6. An image forming apparatus according to claim 1, wherein said imagebearing member is provided in a plural form corresponding to images ofdifferent colors.
 7. An image forming apparatus according to claim 1,further comprising an intermediate transfer member to which an image onsaid image bearing member is transferred.
 8. An image forming methodcomprising the steps of: switching from a first mode for forming animage on an image bearing member at a first speed to a second mode forforming an image on an image bearing member at a speed smaller than thefirst speed; and using reflective surfaces of a reflective polyhedron,in which the number of rotatable surfaces is m, for reflecting lightfrom a light source and optically scanning an image bearing member byskipping n surfaces such that m/(n+1) does not make an integer (providedthat m is an integer of three or more and n is a natural number.).